Substantially all smaller, fractional horsepower electric motors intended for application in the relatively low cost, high volume consumer marketplace have retained classic design features, essentially from their inception decades ago. Most d.c. forms of such motors which are produced at lower cost are of a PM (permanent magnet) variety and are commutated by brushes. In a wide variety of applications, they are powered by batteries. The performance exhibited by these low cost motors leaves much to be desired. Their design results in an inherently poor heat dissipation which, in turn, causes their performance in battery powered consumer devices such as hand tools to be marginal. As more heat is generated due to increased load and current demand, the attendant temperature rise increases the impedance of the motor windings which, in turn, diminishes speed generating capability of the motor. Thus, such motive devices are unable to deliver a continuous high output power performance.
Moreover, brush commutation systems are electrically noisy, precluding their use in conjunction with modern electronic equipment such as computers, and further precluding their use for spark free applications as in the marine field. The classic design for brush commutated PM motors does exhibit, however, the salient aspect of providing a relatively high ratio of the radius from their shaft axis with respect to their working gap to their outer radius. That ratio, typically, is about 0.7 or higher.
The higher ratio is achieved because of a design wherein the laminar pole components about which windings are provided are mounted upon the rotating shaft of the motor. These rotating pole pieces, then extend radially outwardly to the flux working gap in adjacency with outwardly disposed permanent magnets which are mounted adjacent to the outer housing of the motor. The result is an extended radius to the working gap but with the disadvantage of a substantial inability to transfer heat from heat generating coil components on a practical basis through the shaft and bearings to the outside. Fan components have been attached to the shafts but with no practical improvement in motor performance. Of course, the spinning rotor winding requires balancing as part of production. Such typical winding based rotors are characterized by a long average winding turn length that is at least twice as long as the length of the rotor lamination stack. To increase the power of these motors, stack length typically is increased rather than diameter. Thus, the average turn length increases as the length of the rotor stack is increased to increase motor output power resulting in the requirement to increase the diameter of the wire to maintain the motor impedance. The larger diameter wire often results in less turns per pole which, in turn, works against the desired power improvement which was the reason why the stack length was originally increased. This assembly is seen to occur in the conventional designs of both brush commutated and brushless radial gap motors. Notwithstanding their drawbacks, these brush commutated motors prevail in modern consumer products such as hand tools simply because of lower cost.
Over the past, demands did arise for permanent magnet motors commutated by techniques other than the use of brushes. Solid-state electronically commutated or "brushless" small permanent magnet motors entered the market by necessity. For example, brush commutation motors with attendant electrical noise and shorter service life are totally unacceptable as the hard disk drive spindle motor component of a computer. These brushless motors along with the required electronic commutating circuit are expensive to an extent heretofore precluding their use in conventional products intended for consumer markets. However, for applications, such as the computer field, the higher costs are accommodated for out of necessity. In a more conventional design of brushless PM D.C. motors, field windings are positioned about stationary, laminar stator poles which extend inwardly from the outer cover of the motor. The permanent magnet assemblage for the motor is mounted on and rotates with the shaft. An immediate advantage of this arrangement resides in good heat dissipation, providing for stable operation over a wide power range. Additionally, a spinning rotor magnet generally does not require balancing. An immediate disadvantage of the motor resides in its poor power production per unit diameter, i.e., the motor exhibits a relatively low ratio of radius to the working gap from the shaft axis with respect to the radius from the shaft axis to the outer surface of the motor. For motors under 3 inches in diameter, that ratio typically will be about 0.5. This means that the motors inherently exhibit lower power production for their weight and bulk. Further, the conventional designs require longer lamination stacks coincident with longer field winding path lengths which results in heavier weight and an elevated cost for the development of equivalent power.
In response to the highly exacting demands of the computer industry, brushless PM motors have undergone certain levels of refinement. However, such refinements have not mitigated the high costs of the motors, rendering them unacceptable for consumer applications. Typical of such refinements, the subject of detent torque has been addressed in the design of such motors for computers. Inasmuch as the motors are configured having steel core stator poles and associated field windings performing in conjunction with permanent magnets, there occurs a somewhat inherent development of detent torque. Without correction, during an excitation state of the motor windings creating rotational drive, this detent torque will be additively and subtractively superimposed upon the operational characteristics of the motor output to distort the energized torque curve, increase ripple torque, reduce the minimum torque available for starting and, possibly develop instantaneous speed variations. Such instantaneous speed variations generally are uncorrectable by electronics or the like. Another aspect associated with the demands of the computer industry resides in the ever increasing smallness of disk drives attendant with increases in bit densities. PM brushless d.c. motors have been called upon to be fabricated having unusually small outer diameters such that, for example, they may fit within the hub of a computer disk. These motors usually spin the outer surface of the motor diameter upon which the disk drive storage disks are mounted. In this fabrication technique, the motor is constructed similar to conventional PM D.C. brush type motors with the windings on the inside. They also exhibit similar poor heat dissipation characteristics as PM D.C. brush type motors.
Petersen, in U.S. Pat. No. 4,745,345, entitled "D.C. Motor with Axially Disposed Working Flux Gap", issued May 17, 1988, describes a PM D.C. motor of a brushless variety employing a rotor-stator pole architecture wherein the working flux gap is disposed "axially" (parallel with the motor axis) and wherein the transfer of flux is parallel with the axis of rotation of the motor. This "axial" architecture further employs the use of field windings which are simply structured being supported from stator pole core members which, in turn, are mounted upon a magnetically permeable base. The windings positioned over the stator pole core members advantageously may be developed upon simple bobbins insertable over the upstanding pole core members. Such axial type motors have exhibited excellent dynamic performance and efficiency and, ideally, may be designed to assume very small and desirably variable configurations. However the motors are relatively expensive, requiring costly sintered rare earth magnets and the like which, in turn, exhibit a high axial attractive force between the rotor and the stator which is modulated upon the excitation of the field windings creating an undesirable noise level for certain applications such as disk drives.
Petersen, in U.S. Pat. No. 4,949,000, entitled "D.C. Motor", issued Aug. 14, 1990, describes a d.c. motor for computer applications with an axial magnetic architecture wherein the axial forces which are induced by the permanent magnet based rotor are substantially eliminated through the employment of axially polarized rotor magnets in a shear form of flux transfer relationship with the steel core components of stator poles. The dynamic tangentially directed vector force output (torque) of the resultant motor is highly regular or smooth lending such motor designs to numerous high level technological applications (computer disk drives) requiring both design flexibility, volumetric efficiency, low audible noise, and a very smooth torque output. That form of motor also requires the noted sintered permanent magnet structures and other costly materials which preclude the application of the motors to consumer products.
Petersen, et al., in U.S. Pat. No. 4,837,474, entitled "D.C. Motor", issued Jun. 6, 1989, describes a brushless PM d.c. motor in which the permanent magnets thereof are provided as arcuate segments which rotate about a circular locus of core component defining pole assemblies. The paired permanent magnets are magnetized in a radial polar sense and interact without back iron in radial fashion with three core components of each pole assembly which include a centrally disposed core component extending within a channel between the magnet pairs and to adjacently inwardly and outwardly disposed core component also interacting with the permanent magnet radially disposed surface. With the arrangement, localized rotor balancing is achieved and, additionally, discrete or localized magnetic circuits are developed with respect to the association of each permanent magnet pair with a pole assembly. As before, the motor is a specialized device of relatively high cost and intended for the computer industry.
To the present time, there has been no essential breakthrough in the development of brushless PM motors which can contribute their substantial advantage of heat dissipation and electrically quiet control to use with inexpensive consumer products such as hand tools, kitchen implements, and the like.